1. Field of the Invention
The present invention relates to an anti-lock brake control system for a vehicle, comprising: an actuator for regulating the braking pressure for a wheel brake; a wheel speed detecting means for detecting a wheel speed; a wheel acceleration calculating means for differentiating the wheel speed detected by the wheel speed detecting means to provide a wheel acceleration; a vehicle speed calculating means for calculating a presumed vehicle speed based on the wheel speed detected by the wheel speed detecting means; a slip rate calculating means for calculating a slip rate based on the wheel speed detected by the wheel speed detecting means and the presumed vehicle speed determined in the vehicle speed calculating means; a low-pass filter for filtering the wheel acceleration determined in the wheel acceleration calculating means; and an operation control means for controlling the actuator based on at least the slip rate determined in the slip rate calculating means and an output from the low-pass filter during a braking operation to switch-over the reduction, the maintaining and the increasing of the braking pressure from one to another.
2. Description of the Related Art
A system is conventionally constructed as shown in FIG. 4. The actuator 1 for regulating the braking pressure for the wheel brake is driven by an actuator driving means 2, and a control signal from the operation control means 3 is input to the actuator driving means 2. A presumed vehicle speed VR is calculated by the vehicle speed calculating means 5 based on the wheel speed VW detected by the wheel speed detecting means 4, and a slip rate .lambda. is calculated in the slip rate calculating means 6 based on the wheel speed VW and the presumed vehicle speed VR. The wheel speed VW is differentiated in the wheel acceleration calculating means 7, and the wheel acceleration .omega. is filtered in the low-pass filter 8. An output .omega.F from the low-pass filter 8 and the slip rate .lambda. are input to the operation control means 3. A signal indicative of a command to control the operation of the actuation 1 based on the output .omega.F from the low-pass filter 8 and the slip rate .lambda. is output from the operation control means 3.
In such an anti-lock brake control system, the actual vehicle speed V, the wheel speed VW and the presumed vehicle speed VR are varied as shown in FIG. 5A and the slip rate .lambda., the low-pass filter output .omega.F and the braking pressure P.sub.B are varied as shown in FIGS. 5B, 5C and 5D, depending upon the operation of the wheel brake, thereby the wheel avoids falling into a locked state. Specifically, when the slip rate .lambda. becomes larger than a first preset slip rate S0 and the low-pass filter output .omega.F becomes smaller than a first preset acceleration .alpha.0 which is a negative value, pressure-reduction control is started. When the low-pass filter output .omega.F exceeds the first preset acceleration .alpha.0, pressure-reduction control is terminated, a braking-pressure maintaining control is started. Further, when the wheel speed VW starts to be restored, and the slip rate A becomes smaller than a second preset slip rate S1 (( S0) and the low-pass filter output .omega.F becomes smaller than a second preset acceleration .alpha.1, pressure-increase control is started.
The main reason why the low-pass filter output .omega.F is used rather than the wheel acceleration .omega. itself for switching-over the control mode in the operation control means 3 is to avoid a hunting of pressure-increasing.fwdarw.pressure-reduction.fwdarw.pressure-maintaining due to a fine variation in wheel speed due to a road surface condition or a pressure-increase or pressure-reduction pulse. However, there is a possibility that a retard of the termination time point of the pressure reduction and an excessive pressure reduction may be produced, particularly on a road surface having a high coefficient of friction, due to a phase lag of a renewing cycle of the wheel speed or the low-pass filter output .omega.F. If excessive pressure reduction is produced, variation in fluid pressure throughout the anti-lock brake control cycle is increased to bring about a degradation in feeling of the braking operation and a degradation in riding comfort.